GNU CLISP is an ANSI Common Lisp implementation
with an interpreter, compiler, debugger, object
system (CLOS, MOP), sockets, fast bignums, arbitrary precision floats, and a foreign language interface that runs on most Unix variants and Win32.

CMUCL is a free, high performance implementation of the Common Lisp programming language which runs on most major Unix platforms. It mainly conforms to the ANSI Common Lisp standard. CMUCL provides a sophisticated native code compiler; a powerful foreign function interface; an implementation of CLOS; the Common Lisp Object System; which includes multimethods and a metaobject protocol; a source-level debugger and code profiler; and an Emacs-like editor implemented in Common Lisp. CMUCL is maintained by a team of volunteers collaborating over the Internet, and is mostly in the public domain.

Steel Bank Common Lisp is a development
environment for Common Lisp, with excellent
support for the ANSI standard: garbage collection,
lexical closures, powerful macros, strong dynamic
typing, incremental compilation, and the famous
Common Lisp Object System (multimethods and all).
It also includes many extensions, such as native
threads, socket support, a statistical profiler,
programmable streams, and more. These are all
available through an integrated, interactive
native compiler which feels like an interpreter.
SBCL is unique in being a multiplatform native
compiler which bootstraps itself completely from
source, using a C compiler and any other ANSI
Common Lisp implementation.

Jabberwocky is a Lisp IDE containing a Lisp-aware editor with syntax highlighting, parentheses matching, a source analyzer, indentation, a source level debugger, a project explorer, and an interaction buffer. It is the replacement for the Lisp Debug project.

otl is a text processor for generating markup from readable lightweight markup. Much of both the input and output formats can be customized. HTML output is bundled as an example. otl supports complex structures such as nested ordered and unordered lists, headers and footers, and tables.

ACL2 is a mathematical logic, programming
language, and mechanical theorem prover based on
the applicative subset of Common Lisp. It is an
"industrial-strength" version of the NQTHM or
Boyer/Moore theorem prover, and has been used for
the formal verification of commercial
microprocessors, the Java Virtual Machine,
interesting algorithms, and so forth.

CLSQL is an SQL database interface for Common
Lisp. It provides object-oriented and functional
access methods to the underlying database, which
can be one of MySQL, ODBC, PostgreSQL, or SQLite.
It uses the Unified Foreign Function Interface
(UFFI) and thus supports the CMU Common Lisp,
Steel Bank Common Lisp, and Allegro Common Lisp
implementations.

SLIME is an integrated development environment for
Common LISP which does everything you would expect
from an IDE: code evaluation, compilation, macro
expansion, and auto-completion. It also finds
definitions of functions, and marks LISP forms
which the compiler finds to be erroneous. It
provides easy access to implementation-specific
online documentation as well as the ability to
look up symbols in the ANSI Common Lisp HyperSpec.
Further, it includes an interactive debugger and
object inspector.

OpenMCL is an Open Source version of Digitool's excellent Macintosh Common Lisp (MCL) implementation, which runs on LinuxPPC and MacOS X. It features a native code compiler, multithreading support, and good ANSI CL compliance.